Add a new field to obtain the reason code when HTTP error happens while entitlement processsing

Bug: 439954940
Flag: EXEMPT bug fix
Test: atest SatelliteStatsTest PersistAtomsStorageTest
Test: manually run entitlement test using TestEntitlementServer and verfied http status code comes as expected, https://b.corp.google.com/issues/439954940#comment2

Change-Id: I74634dc5406883f78600b1294789667bd8f84d74